Aerial service wire which has been referred to as drop wire extends the communications loop from distribution cable terminals to customers' premises. Typically, telephone plant aerial service wire comprises two parallel spaced metallic conductors. The metallic conductors not only provide transmission paths, but also they provide strength for aerial installations.
In one prior art aerial service wire, for example, it was customary to include two electroformed copper-clad steel conductors enclosed with an extruded rubber insulation covered by a textile serving, and jacketed with a rubber-like material. Although such protective coverings were in widespread use for many years and had proven satisfactory from most standpoints, there developed a long felt desire for an alternative, less expensive covering. Any alternative covering had to have the capability of withstanding exposure to the elements, as well as adequate low temperature flexibility, impact resistance, and abrasion resistance.
A substantially improved plastic covered aerial service wire is disclosed in U.S. Pat. No. 3,935,369 issued on Jan. 27, 1976 to E. J. George, et al. and includes two copper-clad steel conductors enclosed in insulation comprising a plasticized polyvinyl chloride (PVC) composition. The insulation serves also as a jacket. In addition to being more economical, this last described aerial service wire is lighter, more abrasion-resistant and easier to handle and terminate than its predecessor.
In a typical installation, each end portion of the aerial service wire is inserted into a metallic clamp. One of the clamps is attached to a customer's premises and the other one to a pole prior to electrical connections being made to wiring inside the customer's premises and to aerial distribution cable, respectively. At both the customer and the pole end of the aerial service wire, the wire is retained in engagement with the clamp by reactive forces exerted by the clamp on the insulation. These reactive forces come into play as a result of ice and wind loading, for example. If these forces are not transferred from the insulation to the conductors by coupling therebetween, the conductors will slip relative to the insulation within the clamp causing the insulation to rupture and the wire to sag excessively and, in some instances, to fall. Adhesion of the plastic material to the metallic conductors may provide suitable coupling.
On the other hand, if the adhesion between the insulation and the conductors is too great, stripping the covering from the conductors may be difficult. Excessive scraping causes some of the copper from the surface portion of the copper-clad steel conductor to be removed thereby increasing the electrical resistance of the conductor termination. Excessive scraping also reduces the cross-sectional area of the conductor or nicks it, thereby reducing its strength properties to the detriment of its load-supporting capability. Also, scraping may expose the steel, promoting corrosion of the bimetallic conductor.
Although aerial service wires as described in the George, et al. patent are adequate, a new, less expensive design had been sought. Environmental requirements for the disposal of residual copper and processing chemicals have raised the cost of manufacturing copper-clad conductors. Also, these conductors, on occasion, displayed another problem. The problem is the result of the smooth surface of the drawn metallic conductor which does not adhere to the PVC jacket as well as the rough surface of the earlier manufactured electroformed conductor. This problem was solved by applying electrostatically an adhesive to preheated conductors prior to the extrusion of the jacket material, but this solution involves an additional step.
The last described aerial service wire presented some other problems. It is relatively stiff and somewhat difficult to handle. Also, the wire cannot be terminated in insulation displacement type connectors because of the relatively thick PVC insulation which toughens as temperatures decrease and because copper-steel conductors are not easily and reliably accommodated by these kinds of connectors. Further cost reductions of the design are difficult to achieve inasmuch as the PVC material serves as both the jacket and conductor insulation, thereby requiring a balancing of mechanical and electrical requirements.
A most important reason for seeking a new design was the corrosion of the bimetallic conductor which occurs in coastal and industrial areas. Corrosion appears where a conductor has been stripped for termination, where a conductor has been accidentally nicked during separation of the conductors by pliers, or where maintenance tests have been made improperly by inserting test pins through insulation to a conductor.
In a somewhat recently introduced aerial service wire which overcomes these problems, conductors are disposed generally in the geometric center of the cross section transverse to a longitudinal axis of the cable. A jacket which comprises a plastic material encloses the conductors with the conductors being adjacent to each other and to the longitudinal axis of the jacket. The longitudinal axis passes through a geometric center of each transverse cross section of the jacket. This aerial service wire also includes two non-metallic support strands or strength members each of which includes a plurality of filaments that are assembled together and each of which is impregnated with a plastic material which is compatible with that of the jacket. Each of the strength members is disposed between the conductors and an outer surface of the jacket such that the strength members and the longitudinal axis which is interposed therebetween are aligned. Each strength member passes through each cross section of the jacket along an axis which spans the cross section and which extends through the geometric center thereof.
Advantageously, the just-described aerial service transmission medium provides coupling of roving or yarn type strength members to the jacket which avoids prior art schemes of using an intermediate adhesive material or preheating of the conductors which also serve as strength members. Also, because of the geometry, support clamps at a customer's premises and at a pole bear on outer portions of the jacket cross section over the strength members instead of over the conductors. However, it has been found that the coupling of the jacket material to the strength members as determined by a static load test is not as great as had been sought.
What is needed and what still is not provided by the prior art is an aerial service transmission medium which at least meets the strength properties of the most recent prior art aerial service wire. The strength members of the sought-after aerial service transmission medium must have enhanced coupling to the jacket over that of the most recent prior art design and must be compatible with present support schemes at a customer's premises. Still further, the design should be one which easily may be adaptable to optical fiber loop transmission.